Heretofore, as surface treatment for electronic parts which are required to have high reliability, gold film plating has played a center role. However, gold is a material having scarcity value, and accordingly, every time its price sharply rises in succession in market quotations, technical development of alternative metals has attracted attention. Since palladium is inexpensive as compared with gold (bullion) and its metallic density is about 60% of that of gold, palladium has been in the spotlight as an alternative metal for reducing a film thickness of a gold plating film.
However, in highly reliable electronic parts in which not only inexpensiveness but also miniaturization of wiring have been accelerated in recent years, stability and selective deposition properties and reliability of a palladium plating film characteristics have been required.
As an electroless palladium plating solution used for industrial applications, there has heretofore been known, for example, an electroless palladium plating solution comprising a water-soluble palladium salt, ethylenedianimetetraacetic acid, ethylenediamine, and sodium hypophosphite, as described in Patent Document 1.
There has also been known an electroless palladium plating solution containing, as essential components, a palladium compound, at least one member selected from the group consisting of ammonia and an amine compound, an organic compound having divalent sulfur, and at least one member selected from the group consisting of a hypophosphorous acid compound and a boron hydride compound (see, for example, Patent Document 2). From these electroless palladium plating solutions, a palladium-phosphorous alloy plating film is obtained.
In Patent Document 3, there is disclosed, for example, a chemical bath preparation which comprises a palladium salt such as palladium chloride, palladium sulfate, palladium nitrate, palladium acetate or the like, a complexing agent containing one or more nitrogen atoms, and a formic acid or a derivative thereof, and which has a pH value of 4 or higher, and in which no formaldehyde for depositing a palladium layer on a metal surface is present.
Patent Document 4 discloses an electroless palladium plating solution comprising (a) 0.0001-0.5 mol/l of a palladium compound, (b) 0.0005-8 mol/l of at least one member selected from the group consisting of ammonia and amine compounds, and (c) 0.005-5 mol/l of at least one member selected from the group consisting of an aliphatic monocarboxylic acid, an aliphatic dicarboxylic acid, an aliphatic polycarboxylic acid, and water-soluble salts thereof.
The above-mentioned electroless palladium plating solution in Patent Document 1 has not only a drawback of poor storage stability but also a drawback that it is degraded in a short time in an industrial mass production line and thus has a short duration of life as a plating solution. Further, any of plating films obtained from the plating solution are crack-prone and poor in wire-bonding properties and soldering properties, and accordingly, has a difficulty in application to electronic parts.
Moreover, the electroless palladium plating solution disclosed in Patent Document 2 has a drawback that since phosphorus and/or boron derived from the hypophosphorous acid compound and/or the boron compound as a reductive component is contained in a plating film, properties of the palladium plating film markedly change between before and after heat test.
The chemical bath preparation disclosed in Patent Document 3 has not only a drawback of poor storage stability but also a drawback that it is degraded in a short time in an industrial mass production line and thus has a short duration of life as a plating solution.
On the other hand, the palladium plating solution according to Patent Document 4 is a plating solution unsuitable for a highly reliable electronic part having a micro-wiring because anomalous deposition or deposition on other portions than a circuit or electrodes is likely to be caused in such a electronic part due to markedly high deposition rate of palladium in a mass production line. Further, there are technical problems that film thickness control is difficult because palladium plating rate changes with time as the plating solution is continuously used, and that the plating solution has a short duration of life.
Patent Document 1: Japanese Examined Patent Publication No. Sho 46 (1971)-26764
Patent Document 2: Japanese Unexamined Patent Publication No. Sho 62 (1987)-124280
Patent Document 3: Japanese Patent Publication No. 2918339
Patent Document 4: Japanese Unexamined Patent Publication No. Hei 7 (1995)-62549